1. Field of the Invention
The present invention relates to a cordless telephone and particularly to a cordless telephone comprising a master station connected to a telephone line and a plurality of hand sets set to different frequencies for transmission and reception to communicate with the master station by radio.
2. Description of the Prior Art
In the prior art, cordless telephones comprising a master station connected to a telephone line, in which a telephone call can be made by radio communication with the master station are known. Such a cordless telephone is described for example in Japanese Utility Model Publication Gazette No. 7649/1972.
If a number of hand sets having different transmitting and receiving frequencies are provided in such a cordless telephone, it is necessary to transfer a call signal to any one of the hand sets when a master station receives a call signal through a telephone line. In such a case, the master station needs to transfer the call signal to a hand set in the standby mode. If the master station transfers the call signal to a hand set not in the standby mode, the communication cannot be started. In consequence, it is necessary that the master station should be able to identify the operation states of the hand sets. However, in the past, there were no cordless telephones capable of performing such function. Moreover, it is desired these days that a cordless telephone should have multiple functions such as extention calling among the hand sets. If an intercommunication is made among the hand sets with the transmitting and receiving frequencies arranged freely among the hand sets, radio waves of various frequencies are applied in various directions around the master station, which might cause a disturbance in transmission of the radio waves. Accordingly, it is desired that an intercommunication among the hand sets should be made after the permission of the master station and, for this purpose, it is necessary that the master should always watch whether the hand sets request to make an intercommunication. However, in the prior art, there exist no cordless telephones being able to detect the operation states of the hand sets.